1. Field of the Invention
This invention relates to a semiconductor light emitting element and, in particular, to an improvement in electrodes of a semiconductor light emitting element.
2. Description of the Related Art
Conventionally, a gold bump (Au bump) is often used to bond the electrode of a semiconductor light emitting element to a submount etc. However, since the Au bump bonding has many problems such as insufficient heat radiation property, the use of gold-tin solder (AuSn solder) has been proposed instead of the Au bump.
In order to obtain a good bonding property, the top of the electrode of a semiconductor light emitting element is formed of gold and the surface of the electrode except a part of the top surface as a bonding region is covered with a passivation film (protective film). When the semiconductor light emitting element thus constructed is bonded through the AuSn solder to the submount, the gold of the AuSn solder and the electrode may diffuse mutually by heat in the bonding to cause a big deformation of the electrode to allow the peeling or breaking of the passivation film. Also, the Sn originating in the AuSn solder may penetrate or diffuse into the electrode to affect the functions of the element.
JP-A-2003-347487 discloses a method that the electrode is provided with a three-layer structure formed of a titanium (Ti) layer, a nickel (Ni) layer and a gold (Au) or silver (Ag) layer in the order starting from the semiconductor layer side so as to prevent the Sn originating in the AuSn solder from diffusing into the electrode not to allow the peeling of the electrode. However, this method is only a measure after the Sn penetrates into the electrode and cannot prevent the penetration of the Sn into the electrode. Thus, it is ineffective for the Sn to penetrate into the top of the electrode in the bonding to cause the deformation of the electrode. Therefore, in case of having the passivation film formed on the surface of the electrode, the peeling or breaking of the passivation film must be caused by the deformation of the electrode even when using the method. Further, the method cannot prevent the penetration of the Sn into the electrode which results in affecting the functions of the element.
In order to block the penetration/diffusion into the electrode of Sn originating in the AuSn solder to prevent the affectation to the passivation film and the functions of the element, it is required (1) that the Sn is not diffused to the interface between the passivation film and the upper face of the electrode, and (2) that the Sn is not diffused to the lower part of the electrode. An effective measure to meet the requirements is to form a barrier layer on the electrode. The barrier layer needs (1) an enhanced barrier effect, (2) a low fabrication cost, and (3) an excellent mass productivity (or difficulty of peeling). Although it is considered effective to thicken the barrier layer to enhance the barrier effect, the thickened barrier layer may cause a crack or peeling due to stress to reduce the mass productivity. Although the barrier effect may be enhanced by using platinum (Pt) with an excellent barrier effect, the fabrication cost will be significantly increased since the Pt is expensive.